1. Field of the Invention
The present invention relates to a micro rotor, particularly to a micro rotor which is fabricated from an isolated lamination thick film magnet structured of multiple layers each layer including an isotropic magnet with a high remanence Mr and a non-magnetic material to isolate two adjacent isotropic magnets, which has a pole pair number of two or more and which includes a mean magnetic path of in-plane direction having a permeance (B/μoH) of five or more achieved by the magnet alone, and further relates to a rotary electric machine which incorporates such a micro rotor.
2. Description of the Related Art
A rotary electric machine for application in, for example, the field of information and telecommunication devices has been commercially produced with its volume reduced to about 100 mm3 and is widely used. Such a rotary electric machine is requested to be further downsized in order to reduce the size, thickness, weight and power consumption of a drive source of electric and electronic devices or robots for application in the fields of automobiles, home information appliances, communication devices, precision measurement instruments, medical and welfare equipment, and the like.
PCT Patent Application Laid-Open No. H9-501820, for example, discloses a radial gap type brushless DC motor (RG-BLM) with an outer diameter of 1 mm or less and an axial length of 2 mm or less, which includes a hollow circular cylinder having a conductive cylindrical wall with slots and functioning as an excitation winding, and which is applied to an intravascular ultrasonography system. Also, PCT Patent Application Laid-Open No. 2002-532047 discloses a fluid-cooled RG-BLM which has an outer diameter of 8 mm or less and thus can be introduced into a vascular system of a body to thereby drive a blood pump located in the body, and in which an excitation winding is molded by resin containing Al2O3 thereby enhancing heat dissipation performance thus enabling achievement of an output of 5 W at 30,000 rpm.
As an example of the micro rotary electric machine as described above, a brushless DC motor is known which has a volume of 4 mm3 with an outer diameter of 1.6 mm and an axial length of 2 mm wherein a one-pole pair rotor having an outer diameter of 0.76 mm and including an Nd2Fe14B sintered magnet produced by electric discharge machining is coupled to a stator (refer to Non-Patent Document 1). Also known are a brushless DC motor with a volume of 62 mm3 (an outer diameter of 6 mm and an axial length of 2.2 mm) proposed by H. Raisigel (refer to Non-Patent Document 2), further a brushless DC motor with a volume of 20 mm3 (an outer diameter of 5 mm and an axial length of 1 mm) proposed by M. Nakano (refer to Non-Patent Document 3), and still further a brushless DC motor with a volume of 0.6 mm3 (an outer diameter of 0.8 mm and an axial length of 1.2 mm) proposed by T. Ito (refer to Non-Patent Document 4). The rotary electric machines described above undergo a significant decrease in torque due to the volume reduction according to the scaling law.
Various proposals have been presented for a magnet as a micro rotor for use in the rotary electric machines as described above. For example, D. Hinz, et al. introduce a micro rotor made of an Nd2Fe14B system magnet with a thickness of 300 μm, which is die-upset at 750° C. and which has a remanence Mr=1.25 T, a coercive force HcJ=1.06 MA/m, and a (BH)max=290 kJ/m3 (refer to Non-Patent Document 5). Also, J. Delamere, et al. represent that a torque of 0.001 mNm is generated by a motor which includes a micro rotor made of an SmCo system magnet having eight pole pairs and a stator disposed to oppose the rotor component and which is driven at 100,000 rpm, or that an electric power of 1 W is produced by an electric generator structured identically to the motor described above when driven at 150,000 rpm (refer to Non-Patent Document 6). Further, Topfer, T. Speliotis, et al. report a so-called Power MEMS motor adapted to generate a torque of 0.055 mNm and structured to include a micro rotor made of an Nd2Fe14B bonded magnet which is screen-printed on an Fe—Si substrate with a diameter of 10 mm so as to have a thickness of 500 μm, and which has a remanence Mr=0.42 T, and a (BH)max=15.8 kJ/m3 (refer to Non-Patent Document 8).
In terms of torque per volume, that is torque density, of a rotary electric machine, a radial gap type has an advantage over an axial gap type (refer to Non-Patent Document 9). However, a radial gap type rotary electric machine including a slotless iron core suffers an increase in magnetoresistance due to the gap.
Torque is proportional to the number of pole pairs, and mechanical output P (W) is represented by a product of constant k=0.1047 (=π/30), revolution number N (r/min) and torque T (Nm). This suggests that in order to compensate for a decrease of the output P resulting from the miniaturization of a rotary electric machine, it is required that (1) a magnet has a high remanence, (2) magnetization is performed with a high permeance (B/μoH) for two or more pole pairs in the radial direction, and that (3) eddy current loss due to a high speed rotation is reduced.
The micro rotor for the above radial gap type electric rotary machine has a diameter of about 1.6 mm or less. Accordingly, the die-upset magnet of D. Hinz et al. with a remanence Mr=1.25, like an Nd2Fe14B system anisotropic sintered magnet, is magnetically constrained to a C-axis orientation in one single direction. Consequently, the magnetization in the radial direction is limited to one pole pair (=two poles), and also the permeance (B/μoH) cannot be set high because of restriction of magnetic path (configuration). Further, the electric specific resistance of the magnet is low, like nearly 10−5 Ωcm, thus failing to enable suppression of eddy current loss due to a high rotation speed.
On the other hand, the screen-printed Nd2Fe14B bonded magnet of Topfer, T. Speliotis et al. is magnetically isotropic and therefore if the magnet is magnetized with two or more pole pairs in the radial direction and also with more poles than anisotropic magnets fabricated by the die-upset method or the sintering method, a magnetization with a high permeance (B/μoH) is enabled. Moreover, since the screen-printed Nd2Fe14B bonded magnet achieves an electric specific resistance of nearly 10−1 Ωcm, which is comparable to that of a laminated magnetic steel sheet, the eddy current loss due to a high rotation speed can be suppressed. The screen-printed Nd2Fe14B bonded magnet, however, has a remanence of 0.42 T that is lower than that of the anisotropic magnets fabricated by the die-upset method or the sintering method, which results in that in the static magnetic field generated from a micro rotor, a torque produced by a rotary electric machine incorporating the screen-printed Nd2Fe14B bonded magnet is about one third as large as a torque produced by a rotary electric machine which, while having the same figure and structure, incorporates the anisotropic magnet.
With regard to magnetization with two or more pole pairs, for example, H. Komura, et al. report a multi-polar magnetization where an Nd2Fe14B isotropic bonded magnet, which is fabricated such that an Nd2Fe14B isotropic magnetic powder made from a rapidly solidified melt-spun thin ribbon is cured with an epoxy resin and which has a remanence Mr of about 0.62 to 0.68 T, is heated up to 320° C. (Curie Temperature) or higher and then cooled in the magnetic field (Non-Patent Document 9). Though the magnet of H. Kimura, et al has a higher remanence than the example reported by Topfer, T. Speliotis, et al, it is difficult for the radial gap type micro rotary electric machine incorporating the magnet of H. Kimura, et al. to achieve a torque equivalent to or higher than a torque produced by a comparable rotary electric machine incorporating the die-upset or sintered anisotropic magnet. Furthermore, if epoxy resin is to be heated above the Curie Temperature of magnet material that exceeds the decomposition temperature of the epoxy resin, then not only the magnet material applicable must be limited, but also the mechanical strength of magnet deteriorates due to the carbonization of resin component to solidify the magnet material or eddy current loss is increased at a high rotation speed due to the decrease of electric specific resistance. Consequently, the magnet of H. Kimura, et al. is not suitable as a micro rotor in terms of increasing a torque and also achieving an increased output by means of increasing the rotation speed.
Non-Patent Documents which have so far been cited and/or will hereafter be cited are listed as follows:    <Non-Patent Document 1> Mitsubishi Electric Corp. Technical Report-Volume 75 (2001), pp. 703-708, by S. Ohta, T. Obara, Y. Toda and M. Takeda    <Non-Patent Document 2> Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 942-944, by H. Raisigel, O. Wiss, N. Achotte, O. Cugat and J. Delamare    <Non-Patent Document 3> Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 723-726, by M. Nakano, S, Sato, R. Kato, H. Fukunaga, F. Yamashita, S. Hoefinger and J. Fidler    <Non-Patent Document 4> Journal of the Magnetics Society of Japan-Volume 18 (1994), pp. 922-927, by T. Ito    <Non-Patent Document 5> Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 76-83, by D. Hinz, O. Gutfleisch and K. H. Muller    <Non-Patent Document 6> Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 767-778, by J. Delamare, G. Reyne and O. Cugat    <Non-Patent Document 7> Materials for the 143rd Workshop of the Applied Magnetics Society of Japan, Surugadai Kinenkan of Chuo University (2005), by F. Yamashita    <Non-Patent Document 8> Proceedings of the 18th International Workshop on High Performance Magnets and Their Applications, Annecy, France (2004), pp. 942-944, by Toepfer, B. Pawlowski, D. Scha and B. Bel    <Non-Patent Document 9> Journal of Applied Physics-Volume 101 (2007), 09K104, by H. Komura, M. Kitaoka, T. Kiyomiya and Y. Matsuo
It is relatively easy to increase by about 10% the remanence Mr of a micro magnet, for example, the anisotropic magnet of D. Hinz et al. (refer to Non-Patent Document 5) having a remanence Mr of 1.25 T, but the number of pole pair is limited to one and so it is impossible or extremely difficult for the magnet to be magnetized with a high permeance (B/μoH) and also to achieve a high electric specific resistance.
The torque of a rotary electric machine incorporating the above anisotropic magnet can be increased by enhancing the remanence Mr of the magnet, but such a rotary electric machine is disadvantageous in terms of increasing rotation speed due to S-T (Speed-Torque) drooping characteristic. The magnet of D. Hinz et al., particularly, has an electric specific resistance of about 10−5 Ωcm, and therefore it may happen that eddy current is increased due to a high speed rotation and heat energy is generated so as to raise the temperature of the component of a rotor thereby possibly causing thermal demagnetization. Thus, anisotropic magnets fabricated by the die-upset method or the sintering method, which are known to achieve a high remanence Mr, have the technical problems that it is difficult to increase torque by means of increasing the number of pole pairs, also loss is increased when the rotation speed is increased, and furthermore that output is decreased due to demagnetization.
Meanwhile, Topfer, T. Speliotis et al. introduce a rotary electric machine which incorporates a bonded magnet for a micro rotor having an electric specific resistance of nearly 10−1 Ωcm and a remanence Mr of about 0.42 T, whereby eddy current is suppressed so as to achieve a higher rotation speed (refer to Non-Patent Documents 7 and 8). However, such a magnet as described above having a remanence of about 0.42 T generates a static magnetic field that is rather weak for the magnet to be used as a micro rotor incorporated in a radial gap type rotary electric machine usually having a slotless structure with an inherent high reluctance, which raises the technical problem that the above described bonded magnet, even if provided with two or more pole pairs, has a greater tendency to have a torque deficiency than a micro rotor having a remanence Mr of 1.25 T or more
With regard to multi-polar magnetizing a magnet with two or more pole pairs in the radial direction, for example, H. Komura, et al. report a multi-polar magnetization where a bonded magnet, which is fabricated such that an Nd2Fe14B system isotropic magnetic powder made from a rapidly solidified thin ribbon is cured with epoxy resin and which has a remanence Mr of about 0.62 to 0.68 T, has an electric specific resistance of about 102 Ωcm and therefore the problem associated with eddy current can be avoided. But the Nd2Fe14B system magnetic powder and the epoxy resin inevitably suffer thermal degradation during the magnetization process in which they are heated up to 320° C. (Curie temperature) or higher and then cooled in the magnetic field. Also, when part of Fe in Nd2Fe14B is Co-substituted like Nd2(Fe, Co)14B, the Curie temperature is raised by about 10° C. per Co atom %. For example, when about 16 atom % of Fe is Co-substituted, the Curie temperature becomes about 470° C., thus the selection of magnet material is restricted according to the Curie temperature. Further, the mechanical strength as a micro rotor is lowered, and the eddy current loss is increased at a high speed rotation. Moreover, the above bonded magnet having a remanence Mr of 0.62 to 0.68 T, when used as a micro rotor of a rotary electric machine, has the same technical problem as the magnet of Topfer, T. Speliotis et al. that a sufficient torque is not developed as compared with the magnet having a remanence Mr of 1.25 T.
With respect to a micro rotor for the radial gap type rotary electric machine which, as described above, is adapted to achieve a higher torque than an axial gap type rotary electric machine: (1) a magnet structure is scarcely known that is magnetically isotropic, has a high remanence of 0.95 T or more and that has its reluctance minimized in the magnetization direction; also (2) a practical magnet structure is scarcely known in which magnets are isolated in the rotation axis direction and which includes a magnetic path having a permeance (B/μoH) of five or more achieved by the magnet alone, wherein effective magnetic flux is generated dynamically; on the other hand (3) while the number of pole pairs of an anisotropic magnet is restricted to one, the magnet according to the present invention can be provided with two or more pole pairs thereby increasing torque of a resultant rotary electric machine; and further (4) a magnet structure includes a plurality of magnets stacked on one another in the rotation axis direction thereby enabling suppression of eddy current due to a high speed rotation.
There is practically no publicly known technology that can cope properly with the above problems or situations (1) to (4) simultaneously.